30 and 40 what? Certainly not 40 MPH with a surface prop. My Wild thing with a MAC .21 on 60 % nitro, surface prop and very little boat in the water tops out at 47 MPH. The MAC .21 is light years ahead in power compared to the OS.28 that is being considered.
Realistically 20 to 30 MPH is the best that combo will do.